The present invention relates to the fabrication of semiconductor devices and more particularly, to the fabrication of oxide layers on silicon carbide (SiC).
Devices fabricated from silicon carbide are typically passivated with an oxide layer, such as SiO2, to protect the exposed SiC surfaces of the device and/or for other reasons. However, the interface between SiC and SiO2 may be insufficient to obtain a high surface mobility of electrons. More specifically, the interface between SiC and SiO2 conventionally exhibits a high density of interface states, which may reduce surface electron mobility.
Recently, annealing of a thermal oxide in a nitric oxide (NO) ambient has shown promise in a planar 4Hxe2x80x94SiC MOSFET structure not requiring a p-well implant. See M. K. Das, L. A. Lipkin, J. W. Palmour, G. Y. Chung, J. R. Williams, K. McDonald, and L. C. Feldman, xe2x80x9cHigh Mobility 4Hxe2x80x94SiC Inversion Mode MOSFETs Using Thermally Grown, NO Annealed SiO2, xe2x80x9d IEEE Device Research Conference, Denver, Colo., Jun. 19-21, 2000 and G. Y. Chung, C. C. Tin, J. R. Williams, K.
McDonald, R. A. Weller, S. T. Pantelides, L. C. Feldman, M. K. Das, and J. W. Palmour, xe2x80x9cImproved Inversion Channel Mobility for 4Hxe2x80x94SiC MOSFETs Following High Temperature Anneals in Nitric Oxide,xe2x80x9d IEEE Electron Device Letters accepted for publication, the disclosures of which are incorporated by reference as if set forth fully herein. This anneal is shown to significantly reduce the interface state density near the conduction band edge. G. Y. Chung, C. C. Tin, J. R. Williams, K. McDonald, M. Di Ventra, S. T. Pantelides, L. C. Feldman, and R. A. Weller, xe2x80x9cEffect of nitric oxide annealing on the interface trap densities near the band edges in the 4H polytype of silicon carbide,xe2x80x9d Applied Physics Letters, Vol. 76, No. 13, pp. 1713-1715, March 2000, the disclosure of which is incorporated herein as if set forth fully. High electron mobility (35-95 cm2/Vs) is obtained in the surface inversion layer due to the improved MOS interface.
Unfortunately, NO is a health hazard having a National Fire Protection Association (NFPA) health danger rating of 3, and the equipment in which post-oxidation anneals are typically performed is open to the atmosphere of the cleanroom. They are often exhausted, but the danger of exceeding a safe level of NO contamination in the room is not negligible.
Growing the oxide in N2O is possible as described in J. P. Xu, P. T. Lai, C. L. Chan, B. Li, and Y. C. Cheng, xe2x80x9cImproved Performance and Reliability of N2O -Grown Oxynitride on 6Hxe2x80x94SiC,xe2x80x9d IEEE Electron Device Letters, Vol. 21, No. 6, pp. 298-300, June 2000, the disclosure of which is incorporated by reference as if set forth fully herein. Xu et al. describe oxidizing SiC at 1100xc2x0 C. for 360 minutes in a pure N2O ambient and annealing in N2 for 1 hour at 1100xc2x0 C.
Post-growth nitridation of the oxide on 6Hxe2x80x94SiC in N2O at a temperature of 1100xc2x0 C. has also been investigated by Lai et al. P. T. Lai, Supratic Chakraborty, C. L. Chan, and Y. C. Cheng, xe2x80x9cEffects of nitridation and annealing on interface properties of thermally oxidized SiO2/SiC metal-oxide-semiconductor system,xe2x80x9d Applied Physics Letters, Vol. 76, No. 25, pp. 3744-3746, June 2000, the disclosure of which is incorporated by reference as if set forth fully herein. However, Lai et al. concluded that such treatment deteriorates the interface quality which may be improved with a subsequent wet or dry anneal in O2 which may repair the damage induced by nitridation in N2O. Moreover, even with a subsequent O2anneal, Lai et al. did not see any significant reduction in interface state density as compared to the case without nitridation in N2O.
Embodiments of the present invention provide methods for fabricating a layer of oxide on a silicon carbide layer by forming the oxide layer on the silicon carbide layer by oxidizing the silicon carbide layer in an N2O environment. Preferably, a predetermined temperature profile and/or a predetermined flow rate profile of N2O are provided during the oxidation. The predetermined temperature profile and/or predetermined flow rate profile may be constant or variable and may include ramps to steady state conditions. The predetermined temperature profile and/or the predetermined flow rate profile are selected so as to reduce interface states of the oxide/silicon carbide interface with energies near the conduction band of SiC.
In particular embodiments of the present invention, the predetermined temperature profile may result in an oxidation temperature of at least about 1200xc2x0 C. In particular embodiments, the oxidation temperature is about 1300xc2x0 C. In further embodiments of the present invention, the duration of the oxidation may vary depending on the thickness of the oxide layer desired. Thus, oxidation may be carried out for from about 15 minutes to about 3 hours or longer.
In additional oxidation embodiments of the present invention, the predetermined flow rate profile includes one or more flow rates of from about 2 Standard Liters per Minute (SLM) to about 6 SLM. In particular embodiments, the flow rates are from about 3.5 to about 4 Standard Liters per Minute.
In further embodiments, formation of the oxide layer may be followed by annealing the oxide layer in inert gas such as Ar or N2 or combinations thereof. The post formation anneal may also be carried out in a hydrogen containing environment, such as H2 or combinations of H2 and one or more inert gases such as Ar or N2. Such an annealing operation may be carried out, for example, for about one hour.
In still further oxidation embodiments of the present invention, the predetermined flow rate profile provides a velocity or velocities of the N2O of from about 0.37 cm/s to about 1.11 cm/s. In particular embodiments, the predetermined flow rate profile provides a velocity or velocities of the N2O of from about 0.65 cm/s to about 0.74 cm/s.
In additional oxidation embodiments, methods for fabricating a layer of oxide on a silicon carbide layer include forming the oxide layer on the silicon carbide layer in an N2O environment at a predetermined temperature profile which includes an oxidation temperature of at least about 1200xc2x0 C. and at a predetermined flow rate profile for the N2O. The predetermined flow rate profile may be selected to provide an initial residence time of the N2O of at least about 11 seconds.
In particular oxidation embodiments of the present invention, the initial residence time may be from about 11 seconds to about 33 seconds. In still further embodiments of the present invention, the initial residence time is from about 19 seconds to about 22 seconds.
Additionally, a total residence time of the N2O may be from about 28 seconds to about 84 seconds. In such oxidation embodiments of the present invention, the total residence time may also be from about 48 seconds to about 56 seconds.